The present invention relates to a method for the continuous production of pulp from fibrous, lignocellulosic materials by defibering and/or refining the fibrous material between refining discs that are rotatable relative to each other, in a steam atmosphere at an elevated temperature and at a pressure which may be greater or less than atmospheric pressure. The fibrous material is treated in the presence of alkali, and substances are added to the material having a chemical action thereon, such as "per" compounds, especially peroxides, which are used as bleaching agents for their mild effect on lignin. On the other hand, peroxides have a tendency to decompose, e.g. under the influence of heat, and for this reason the effectiveness of the bleaching agent deteriorates when the fibrous material, to which the bleaching agent is added, is subjected to lengthy steam treatment. A lengthy period of steam treatment also makes the fibrous material harder to bleach.
The bleaching of mechanical pulps by means of, e.g., hydrogen peroxide, is thus a known process, but bleaching by this process has hitherto been carried out at temperatures up to approximately 60.degree. C., where the decomposition of the peroxide begins to be appreciable. At temperatures of 100.degree. C. and above, the rate of decomposition is so rapid that effective bleaching in a bleaching tower by conventional methods cannot be achieved. Refiner mechanical pulps have been successfully bleached in a disc refiner at relatively high concentrations, 10% to 20%, in conjunction with so-called post-refining, which requires comparatively little energy, with the result that the rise in temperature is moderate. A disc refiner is a perfect mixer for achieving rapid and thorough blending of the pulp and the liquors, and it does this fairly independently of the concentration of the pulp. The concentration must often be lower than stated above, however, to prevent an injurious rise in temperature.
In consequence of the brief period that the pulp is in the refiner and of the moderately high temperature, approximately 60.degree. C. to 80.degree. C., it is not usually possible to utilize the full bleaching capacity of the peroxide in the refiner, but the pulp must be allowed to stand for a period of time depending on the amount of residual peroxide in the pulp to undergo an after bleaching step after it has passed through the refiner.
In the production of so-called thermomechanical pulps (TMP) with chips in a disc refiner, experiments have been made with adding peroxide to the chips, i.e., feeding in the peroxide via the refiner intake pipe. So far, however, this has had little success, as is confirmed by an article in Pulp & Paper Canada, March 1976, page 63, which, among other things, reports experiments with the addition of peroxide solution in conjunction with pressure defibration, under various conditions, e.g., before and after defibration, in the production of thermomechanical pulp. A summary of results recommends as the best solution the addition of the peroxide solution after the pressure stage, so that the resultant pulp will be bleached under less stringent temperature conditions than those prevailing during the pressure stage. It is probable that earlier attempts to bleach the fibrous material in conjunction with pressure defibration at temperatures above 100.degree. C. have failed to lead to the desired result because of the extremely rapid rate of decomposition of the peroxide, with consequent waste of the bleaching effect. Thus, in the production of bleached pulps of the type in question herein, it has appeared to the expert heretofore that the best procedure is to bleach the pulp in a separate processing stage from defibering, as described above.